The present invention generally relates to electronic amplifiers, and more particularly to a method of operating a distributed amplifier having separately biased amplifier sections.
Distributed amplifiers and mixers have been used extensively for many years in a variety of broadband system applications such as microwave receivers, wide-band transmitter exciters and low noise oscilloscope preamplifiers. Distributed amplifiers are conventionally configured to employ multiple amplifier cells within the distributed transmission line networks. The conventional configuration of multiple amplifier cells within the distributed transmission line networks generally exhibits the desired increase in gain. However, the efficiency of the conventional configuration of multiple amplifier cells within the distributed transmission line networks degrades with a reduction in drive power.
Referring to FIG. 1, a distributed amplifier 10 is illustrated according to the prior art. The distributed amplifier 10 is shown with multiple cells (12,14,16). Each of the cells (12,14,16) includes a field effect transistor (FET) 18. The drain terminal 20 of the FET 18 of each of the cells (12,14,16) are coupled with output-line inductances 22, which are connected to an output-line ground 24 with an output-line termination resistance 26. The gate terminals 28 of the first FET 18 of each of the cells (12,14,16) are coupled with input-line inductances 30 that are connected to an input-line ground 32 with an input-line termination resistance 34.
The distributed amplifier 10 of the prior art as illustrated in FIG. 1 has a lower output with a reduction in the drive power. However, the Direct Current (DC) power consumption is not substantially reduced so that the Power-Added Efficiency (PAE) of the distributed amplifier 10 of the prior art degrades as a function of the output power (Pout) An example of PAE degradation as a function of the output power (Pout) for a distributed amplifier of the prior art is illustrated in the graph of FIG. 2.
Distributed amplifiers of the prior art, such as the distributed amplifier 10 shown in FIG. 1, that are designed to operate from about two to twenty Giga-Hertz (GHz) (i.e., microwave amplifiers) have been fabricated on gallium arsenide (GaAs) substrates. These microwave amplifiers that are fabricated on GaAs substrates have circuit elements with relatively small values, which require minimal space on the GaAs substrate (e.g., inductors of one nH to two nH typically require an area of fifteen microns by fifteen microns). However, if the distributed amplifier is designed for frequencies below about three GHz, numerous circuit elements are used with values that require a larger space on the GaAs substrate than the circuit elements used for distributed amplifiers designed for frequencies greater than about ten GHz (e.g. an output-line inductances 24 of ten nH would generally require an area of sixty microns by sixty microns.) Therefore, the distributed amplifiers that are designed for frequencies below about ten GHz on GaAs substrates tend to utilize an undesirable amount of semiconductor material that reduces the cost effectiveness of such a device.
In view of the foregoing, it should be appreciated that it would be desirable to increase the cost effectiveness of a distributed amplifier and more preferably to increase the cost effectiveness of a distributed amplifier that is designed for frequencies below about twenty GHz, more preferably below ten GHz, even more preferably below about five GHz, and most preferably below about two GHz. In addition, it is desirable to provide a linear distributed amplifier with a PAE that is not substantially affected by a reduction in the drive power. Furthermore, additional desirable features will become apparent to one skilled in the art from the drawings, foregoing background of the invention and following detailed description of a preferred exemplary embodiment, and appended claims.